Multirotor aircraft with a thrust producing unit that comprises an aerodynamically optimized shrouding

ABSTRACT

A multirotor aircraft with an airframe that extends in a longitudinal direction, and with at least one thrust producing unit for producing thrust in a predetermined thrust direction, wherein the at least one thrust producing unit comprises a shrouding that is associated with at least one rotor assembly comprising at least one electrical engine, wherein the shrouding defines a cylindrical air duct that is axially delimited by an air inlet region and an air outlet region, wherein a cantilever is mounted at a leading edge region of the cylindrical air duct to the shrouding such that the cantilever is arranged inside of the cylindrical air duct and oriented at least essentially in parallel to the longitudinal direction, wherein the shrouding comprises a forward beam which connects the cantilever to the airframe, the forward beam being arranged outside of the cylindrical air duct and comprising a forward flange that is rigidly attached to the airframe, wherein the at least one electrical engine is mounted to the cantilever, and wherein the cylindrical air duct is provided in opened perimeter configuration, the shrouding being at least partly cut-off in the opened perimeter configuration at a trailing edge region of the cylindrical air duct over a predetermined opening angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/438,548, filed Jun. 12, 2019, which claims priority to Europeanpatent application No. EP 18400014.9 filed on Jun. 13, 2018, thedisclosures of which are incorporated in their entirety by referenceherein.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The invention is related to a multirotor aircraft with an airframe thatextends in a longitudinal direction, and with at least one thrustproducing unit for producing thrust in a predetermined thrust direction,wherein the at least one thrust producing unit comprises a shroudingthat is associated with at least one rotor assembly.

(2) Description of Related Art

Various conventional multirotor aircrafts are known, e.g. from thedocuments EP 2 551 190 A1, EP 2 551 193 A1, EP 2 551 198 A1, EP 2 234883 A1, WO 2015/028627 A1, U.S. Pat. No. 3,262,657, US D678 169 S, U.S.Pat. Nos. 6,568,630 B2, 8,393,564 B2, 7,857,253 B2, 7,946,528 B2,8,733,690 B2, US 2007/0034738 A1, US 2013/0118856 A1, DE 10 2013 108 207A1, GB 905 911, CN 202728571 U and CN 201306711 U. Other multirotoraircrafts are also known from the state of the art, such as e.g. theBoeing CH-47 tandem rotor helicopter, the Bell XV-3 tilt rotor aircraft,the Bell XV-22 quad tilt with ducted rotors, as well as so-called dronesand, more particularly, so-called quad drones, such as e.g. described inthe documents US 2015/0127209 A1, DE 10 2005 022 706 A1 and KR 101 451646 B1. Furthermore, multirotor aircraft studies and fictions alsoexist, such as e.g. the crossover-mobility vehicle Pop Up. Next that wasdeveloped by Airbus, Italdesign and Audi, the autonomous aerial vehicleEhang 184 that was developed by Bejing Yi-Hang Creation Science &Technology Co. Ltd., the multicopter Volocopter VC200 that was developedby e-Volo GmbH, the vertically taking off and landing (VTOL) aircraft S2that was developed by Joby Aviation Inc., the skyflyer SF MK II that wasdeveloped by Skyflyer Technology GmbH, and the multicopter shown in theAvatar movie.

Each one of these conventional multirotor aircrafts is equipped with twoor more thrust producing units that are provided for producing thrust ina predetermined thrust direction during operation of the multirotoraircraft. In general, each thrust producing unit includes one or morerotors or propellers and is, usually, designed for specific flightconditions. By way of example, a thrust producing unit that is designedas an airplane propeller operates at its optimum in cruise conditions,whereas a thrust producing unit that is designed as propeller of acompound helicopter is essentially adapted for hover or forward flightconditions, while a thrust producing unit that implements e.g. aso-called Fenestron® tail rotor is particularly designed for hoverconditions.

In all of these examples, the respective thrust producing unit isoptimized for operation in axial air flow conditions, i.e. in an airflow direction that is oriented at least approximately along a rotoraxis resp. rotation axis of the given one or more rotors or propellersand, therefore, referred to as an axial air flow direction. If, however,the respective thrust producing unit is operated in transversal air flowconditions, i.e. in an air flow direction that is oriented transverse tothe rotor axis of the given one or more rotors or propellers and,therefore, referred to as a non-axial air flow direction, a respectiveefficiency of the thrust producing unit usually decreases considerably.

By way of example, in the case of operation of a multirotor aircraftwith two or more thrust producing units, the thrust producing units willbe subjected to axial air flow conditions e.g. during a verticaltake-off phase. Subsequently, respective thrust vectors generated by thethrust producing units can be inclined in a predetermined direction,e.g. by rotating the thrust producing units accordingly, so that themultirotor aircraft gains velocity and leaves a previous hoveringcondition such that is converts to forward flight, wherein the thrustproducing units are subjected to transversal air flow conditions.However, in the transversal air flow conditions, respective ducts orshrouds, which are beneficial in axial air flow conditions, arepenalizing by generating a comparatively large amount of drag. In otherwords, an underlying advantage provided by the ducts or shrouds inhovering turns out to be a disadvantage in forward flight, whichincreases with increasing a respective advancing speed of the multirotoraircraft in forward flight.

Nevertheless, it should be noted that in axial air flow conditions aducted rotor or propeller, i.e. a rotor or propeller that is providedwith a duct or shroud, is approximately 25% to 50% more efficient thanan equivalent isolated or non-ducted rotor or propeller, i.e. a rotor orpropeller without duct or shroud, which has comparable globaldimensions, i.e. diameter and mean chord. In other words, the presenceof a duct or shroud increases a respectively produced thrust of a giventhrust producing unit at constant required power. Therefore,conventional thrust producing units are frequently provided with one ormore rotors or propellers that is/are completely enclosed in anassociated duct or shroud. This classical configuration uses arespective rotor or propeller induced velocity to generate thrust alsofrom the duct or shroud.

In general, a duct or shroud is defined by an enclosed, annular surfacethat is arranged around a rotor or propeller in order to improverespective aerodynamics and performances of the rotor or propeller. Aconventional duct or shroud is usually not rotatable, i.e. cannot beinclined, and has a height that is selected such that a given rotor orpropeller is fully enclosed therein.

However, as the duct or shroud must have a certain height or length inorder to enclose an associated rotor or propeller and is, thus,comparatively large in size, the duct or shroud increases an overallweight of a respective multirotor aircraft due to its size, and furtherincreases aerodynamic drag e.g. during forward flight, i.e. intransversal air flow conditions, as the duct or shroud cannot beinclined for adjustment of an underlying thrust vector direction.Increase of the aerodynamic drag is particularly critical for multirotoraircrafts that are provided with electrically powered engines, as theaerodynamic drag must be considered when sizing a respectively requiredelectrical drive system and, further, it directly impacts an achievableflight time.

In other words, although generation of additional thrust is an importantadvantage resulting from the use of the duct or shroud, the duct orshroud is, however, strongly penalizing in forward flight, i.e. intransversal air flow conditions, due to additional aerodynamic draggenerated by the duct or shroud. The additional aerodynamic drag isdirectly proportional to a respective frontal area that is defined by aproduct of height and width of the duct or shroud.

Therefore, considerable efforts are made to improve design and, thus,underlying aerodynamics and performances of conventional ducts orshrouds. Usually, such efforts merely concentrate on the ducts orshrouds as such, while associated structural supports that are requiredfor either attaching required engines within the ducts or shrouds, orfor mounting the ducts or shrouds to a respective airframe of amultirotor aircraft, are not taken into consideration. However, theseassociated structural supports and the required engines cause usually upto 70% of an overall aerodynamic drag of the respective multirotoraircraft, while its airframe, which occupies a significantly largerarea, barely contributes to this overall aerodynamic drag.

Furthermore, other design requirements for ducts or shrouds must also betaken into consideration in addition to the aerodynamic drag. Inparticular, an underlying aerodynamic lift and resulting static loads,an accuracy of shape, underlying Eigenmodes resp. dynamic stiffness,noise generation, as well as engine and structural integration must beconsidered.

More particularly, with respect to aerodynamic lift and resulting staticloads, respective ducts or shrouds may be configured such that they aresuitable to create an important amount of lift on their respectiveleading edge regions. However, such huge amount of lift inducessignificant bending moments at the leading edge regions. Suchsignificant bending moments, however, lead to significant deformationsat the leading edge regions which are in conflict with an underlyingrequirement of close tolerance between the ducts or shrouds and a rotoror propeller that is enclosed therein. In order to avoid suchsignificant deformations, a given duct or shroud can be covered by arespective shear material having a suitable thickness, or by includingadditional stator vanes within the ducts or shrouds. However, suchdesign measurements are contrary to a required lightweight design orreduction of noise generation.

With respect to the accuracy of shape, an underlying design mustconsider that ducts or shrouds are generally close tolerance parts. Morespecifically, the smaller underlying tolerances are designed, the betterare obtainable beneficial effects on air flow and performance of a givenduct or shroud. A typical size of a realized gap between respectiverotor blades of a rotor or propeller that is enclosed in the duct orshroud is about 4 mm between the blade tips and the duct or shroud. Ingeneral, the accuracy of shape is ensured by associated stator vanes.However, it is well-known that the number of provided stator vanes isdetrimental on the aerodynamic drag and noise generation of themultirotor aircraft.

With respect to the Eigenmodes and dynamic stiffness, it must beconsidered that the above-mentioned structural supports that attach theducts or shrouds to associated airframes of multirotor aircrafts exhibitan unfavorable vibrational behavior due to a respectively underlyinglength of the structural supports that are usually supported on only oneside of the ducts or shrouds. This, however, represents significantdrawbacks with respect to weight, flight comfort and fatigue loads ofrespective multirotor aircrafts.

With respect to noise generation, it must be taken into considerationthat this is an important criterion if a respective multirotor aircraftis intended for use in transportation of passengers, e.g. in use as anair taxi. In other words, for such air taxis respective authorities willrequest particular configurations with reduced noise generation. In thisrespect, it must be noted that the structural supports required forattaching the ducts or shrouds to a respective airframe of a multirotoraircraft and/or for attaching required engines within the ducts orshrouds are by definition crossing a generated downwash of associatedrotors or propellers and are, hence, a source of noise. This isparticularly critical if these structural supports extend radially to arotor hub of the engines.

Finally, with respect to the engine and structural integration, it mustbe noted that integration of the engine is always complicated as manysecondary requirements, such as accessibility and maintainability, mustbe ensured. On the other hand, structural and fatigue strength,stiffness, aerodynamic drag and the possibility of cooling integrationmust be guaranteed. However, in conventional designs integration of acooling, such as e.g. a heat exchanger, is generally very complicated.

The document US 2012/0012692 A1 describes a multirotor aircraft having aplurality of thrust producing units. On each side of this multirotoraircraft, a thrust producing unit with four vertical lift rotors isarranged within a protective shroud, which is also referred to as afence. Each vertical lift rotor is mounted to a propulsion boom that isarranged at a central position of the shroud and oriented at leastessentially in parallel to a longitudinal extension of the multirotoraircraft. More specifically, a single propulsion boom is mounted insideof each shroud and supports all four vertical lift rotors that areassociated therewith. Furthermore, each one of the propulsion booms isattached to an airframe of the multirotor aircraft by means of threeassociated struts, which also mount the shroud to the airframe. Thevertical lift rotors may be provided with associated electrical engines.However, in this multirotor aircraft the vertical lift rotors are merelyintended for lifting the aircraft, but they are not intended tocontribute to forward flight and, as such, they are not required tooperate in transversal air flow conditions.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a newmultirotor aircraft that exhibits improved aerodynamics and performancesin transversal air flow conditions.

This object is solved by a multirotor aircraft comprising the featuresof claim 1. More specifically, according to the present invention amultirotor aircraft with an airframe that extends in a longitudinaldirection, and with at least one thrust producing unit for producingthrust in a predetermined thrust direction, is provided. The at leastone thrust producing unit comprises a shrouding that is associated withat least one rotor assembly comprising at least one electrical engine.The shrouding defines a cylindrical air duct that is axially delimitedby an air inlet region and an air outlet region. A cantilever is mountedat a leading edge region of the cylindrical air duct to the shroudingsuch that the cantilever is arranged inside of the cylindrical air ductand oriented at least essentially in parallel to the longitudinaldirection. The shrouding comprises a forward beam which connects thecantilever to the airframe. The forward beam is arranged outside of thecylindrical air duct and comprises a forward flange that is rigidlyattached to the airframe. The at least one electrical engine is mountedto the cantilever. The cylindrical air duct is provided in openedperimeter configuration, and the shrouding is at least partly cut-off inthe opened perimeter configuration at a trailing edge region of thecylindrical air duct over a predetermined opening angle.

It should be noted that the term “shrouding” should be understood asencompassing simultaneously the terms “duct” and “shroud”. In otherwords, in the context of the present invention, the term “shrouding”refers interchangeably to a duct or a shroud.

Advantageously, the at least one thrust producing unit of the inventivemultirotor aircraft is implemented as a shrouded multiple rotor assemblyconfiguration that leads to a significantly reduced aerodynamic drag intransversal air flow conditions, e.g. in forward flight of the inventivemultirotor aircraft. This significantly reduced aerodynamic drag resultsat least partially from a preferred design of the shrouding itself, inparticular from an underlying undulated geometry of the air inlet regionin circumferential direction of the cylindrical air duct.

More specifically, the shrouding of the at least one thrust producingunit and all associated elements are preferably axially non-symmetric,i.e. non-symmetric over the azimuth ψ of the shrouding. In other words,the shrouding is designed on the basis of a variable factor with respectto all associated elements, i.e.:

-   -   Height vs. Azimuth ψ,    -   Air inlet region radius vs. Azimuth ψ,    -   Air outlet region radius vs. Azimuth ψ, and/or    -   Arrangement of additional lifting surfaces vs. Azimuth ψ.

In particular, a variable height of the shrouding enables significantadvantages in the trade-off between vertical take-off and hovering,wherein an underlying efficiency increases with an increase of theheight of the shrouding, and forward flight, wherein an underlying dragdecreases with a decrease of the height of the shrouding, as thisreduces a respective drag area of the shrouding.

Preferably, the shrouding is used as an additional lifting device duringhover and forward flight cases of the inventive multirotor aircraft and,thus, beneficially allows reduction of a respective power consumption ofthe inventive multirotor aircraft. Furthermore, the shrouding mayprovide for a shielding effect with respect to a rotor assembly that isaccommodated therein and, thus, advantageously allows to reduce arespective rotor noise footprint on ground.

According to one aspect, the at least one thrust producing unit can beprovided with a foreign object protection, e.g. by being enclosed by agrid, in order to protect a rotor assembly that is accommodated thereinfrom foreign objects. Such a foreign object protection beneficiallyprevents misuse and accidents by and of individuals, e.g. by preventingthem from getting their hands caught in rotating parts, thereby leadingto an increased operational safety level of the at least one thrustproducing unit of the inventive multirotor aircraft.

Advantageously, by providing the at least one thrust producing unit ofthe inventive multirotor aircraft with the at least two rotor assembliesthat define different rotor planes, the rotor assemblies can bepositioned above each other and rotated in a counter rotating manner,leading to a thrust producing unit that provides for an increased safetylevel and that allows reduction of the overall dimensions of theinventive multirotor aircraft, resulting in a comparatively smallaircraft, since the two or more rotor planes can be combined in a singlethrust producing unit.

According to one aspect, the shrouding of the at least one thrustproducing unit of the inventive multirotor aircraft with the cylindricalair duct relieves the electrical engine and beneficially createsadditional lift. This is important in order to increase an underlyingefficiency of the inventive multirotor aircraft and its electricalengine on a global level.

An important aspect of the present invention consists in a suitablestructural integration of the shrouding and, thus, the at least onethrust producing unit into the inventive multirotor aircraft. Morespecifically, the at least one thrust producing unit preferablycomprises a load carrying framework that comprises the carrier beamwhich is used for mounting of the at least one electrical engine. Thiscarrier beam advantageously transfers loads from the at least oneelectrical engine to associated forward and aft beams of the at leastone thrust producing unit and is stiffening the cylindrical air ductresp. the shrouding in longitudinal direction of the airframe of themultirotor aircraft in order to prevent ovalisation in operation bysupporting the cylindrical air duct at its highest loaded locations.Furthermore, due to its longitudinal orientation in parallel to alongitudinal extension of the airframe of the inventive multirotoraircraft, the carrier beam merely contributes in a completelyneglectable manner to an overall aerodynamic drag of the inventivemultirotor aircraft. In particular, as it is not arranged radially withrespect to rotor blades of a given rotor assembly, it exhibitssignificant advantages concerning its contribution to noise emission.

Furthermore, the forward and aft beams that are connected to the carrierbeam, preferably attach the shrouding of the at least one thrustproducing unit to the airframe of the inventive multirotor aircraft. Theforward and aft beams may be provided with continuous flanges in orderto be as stiff as possible with respect to bending loads. Furthermore,if torsional loads must be taken into consideration and are important,closed profiles can be used for a respective support structure that isdefined by the forward and aft beams, respectively their continuousflanges.

Preferably, the forward and aft beams are located outside of thecylindrical air duct. Thus, they can advantageously be shapedaerodynamically by any means without detrimental effect on the downwashof the at least one thrust producing unit. Preferably, the forward andaft beams are integrated into the shrouding of the at least one thrustproducing unit.

According to one aspect, implementation of the at least one thrustproducing unit with a shrouding as described above advantageously allowsfor integration of the electrical engine and cooling. More specifically,due to the longitudinally arranged carrier beam inside of thecylindrical air duct, the at least one electrical engine can beintegrated eccentrically, i.e. sideward of the carrier beam. This allowsthe carrier beam to work as a continuous bending beam without anytapering and cross section variances. This has a significant advantagewith respect to stiffness, stress, and fatigue. Furthermore, the carrierbeam offers a large area which is also oriented along a transversal airflow direction and, thus, a large cooling area for the electrical engineis provided and available, which is, however, not contributing to theoverall aerodynamic drag of the inventive multirotor aircraft.

Furthermore, the shrouding of the at least one thrust producing unit canbe optimized with respect to aerodynamics and performance in that it canbe designed in order to provide additional lift. In particular, aleading edge region of the shrouding can be designed for providing suchadditional lift.

According to one aspect, the carrier beam is arranged eccentricallyinside of the cylindrical air duct and at least essentially coplanar toa cross section of the cylindrical air duct.

According to another aspect, the carrier beam is a cantilever.

According to still another aspect, the carrier beam is further mountedat a trailing edge region of the cylindrical air duct to the shrouding.

According to still a further aspect, the carrier beam is bar-shaped andextends from the leading edge region to the trailing edge region.

According to still another aspect, the shrouding comprises a forwardbeam and an aft beam which both connect the carrier beam to theairframe, the forward beam and the aft beam being arranged outside ofthe cylindrical air duct.

According to still another aspect, the forward beam comprises a forwardflange, wherein the aft beam comprises an aft flange, the forward flangeand the aft flange being attached to the airframe.

According to still another aspect, the forward beam and the forwardflange are integrally formed, and the aft beam and the aft flange arelikewise integrally formed. Preferably, the forward flange and the aftflange are integrally formed.

According to still another aspect, the cylindrical air duct is providedin closed perimeter configuration or in opened perimeter configuration.The shrouding is at least partly cut-off in the opened perimeterconfiguration at a trailing edge region of the cylindrical air duct overa predetermined opening angle.

According to still another aspect, the shrouding is provided with anadditional lifting surface at the leading edge region of the cylindricalair duct.

According to still another aspect, the at least one electrical engine iseccentrically mounted to the carrier beam.

According to still another aspect, the air inlet region exhibits incircumferential direction of the cylindrical air duct an undulatedgeometry. The cylindrical air duct comprises in circumferentialdirection a leading edge region and a diametrically opposed trailingedge region, a board side lateral region and a diametrically opposedstar board side lateral region, wherein the board side lateral regionand the star board side lateral region are respectively arranged in thecircumferential direction of the cylindrical air duct between theleading edge region and the trailing edge region. A height at theleading edge region is preferably smaller than a height at the boardside lateral region and/or the star board side lateral region.

According to still another aspect, the cylindrical air duct exhibits aheight defined between the air outlet region and the air inlet region inaxial direction of the cylindrical air duct that varies incircumferential direction of the cylindrical air duct. The height thatvaries in the circumferential direction of the cylindrical air ductdefines the undulated geometry of the air inlet region.

According to still another aspect, the height at the trailing edgeregion is smaller than a height at the board side lateral region and/orthe star board side lateral region.

According to still another aspect, the height at the trailing edgeregion is smaller than the height at the leading edge region.

Advantageously, the shrouding of the at least one thrust producing unitof the inventive multirotor aircraft allows reducing respective overalldimensions of the inventive multirotor aircraft. Furthermore,individuals approaching the shrouded thrust producing unit are protectedagainst injury. Moreover, foreign object damages of the thrust producingunit in operation, such as e.g. bird strike or wire strike, can securelyand reliably be prevented. In addition, the overall operational safetyof the inventive multirotor aircraft in case of air collisions can beimproved.

Moreover, respective aerodynamics, acoustics and performances can beimproved by reducing a respective rotor blade loading in operation,reducing an overall power consumption, reducing a respective noiseemission and ameliorating functioning in hover and forward flight of theinventive multirotor aircraft. Furthermore, an underlying requireddiameter of the at least one thrust producing unit can be reduced.Additionally, lift of the inventive multirotor aircraft is improved bythe shrouding itself, potentially reducing the overall power required bythe inventive multirotor aircraft.

It should be noted that although the inventive aircraft is describedabove with reference to a multirotor structure with multiple rotorassemblies, it could likewise be implemented as a multipropellerstructure with multiple propeller assemblies or as a multipropeller and-rotor structure. More specifically, while rotors are generally fullyarticulated, propellers are generally not articulated at all. However,both can be used for generating thrust and, thus, for implementing thethrust producing unit of the multirotor aircraft according to thepresent invention. Consequently, any reference to rotors or rotorstructures in the present description should likewise be understood as areference to propellers and propeller structures, so that the inventivemultirotor aircraft can likewise be implemented as a multipropellerand/or multipropeller and -rotor aircraft.

In other words, the present invention principally relates to a multiplethrust configuration with rotors/propellers that define rotor/propellerplanes, which can be selected to be positioned atop of each otherindividually, a shrouding for enclosing any rotating parts of at mostone of the rotors/propellers, at least one electrical engine whichdrives each rotor/propeller, wherein each engine can be segregated inorder to increase a provided safety level, and wherein a logicconnection preferably exists between battery and electrical engines, thelogic connection preferentially comprising a redundant design increasingthe safety level in case of failure, and wherein preferably a batteryredundancy layout with an appropriate safety level in case of failure isprovided.

Advantageously, by providing the at least one thrust producing unit ofthe inventive multirotor aircraft as described above, the aerodynamicdrag that is generated by the carrier beam that carries the at least oneelectrical engine of the rotor assembly can be eliminated almostcompletely. A respective aerodynamic drag which is still produced by theat least one electrical engine may advantageously be minimized, asinstallation of a smooth aerodynamic attachment may easily beintegrated. Furthermore, the aerodynamic drag that is produced on theshrouding itself is beneficially supported at a respective locationwhere its majority is created such that deformation and ovalisation ofthe cylindrical air duct may advantageously be prevented. Furthermore,the carrier beam's geometric shape is preferably adapted to associatedstatic requirements, i.e. the carrier beam's bending around a globallateral resp. transversal axis, and results in implementing the carrierbeam higher in vertical direction than wider in lateral resp.transversal direction. Consequently, the carrier beam's geometric shapeis no longer impacting the aerodynamic behavior significantly.

Furthermore, it should be noted that the majority of lift is created ata respective leading edge region of the shrouding. Advantageously, acorresponding load is directly supported and occurs on this leading edgeregion. More specifically, due to a preferred duct design in the leadingedge region in terms of design space, the inventive support structureexhibits enough inertia to take the loads without notable bending ortorsional deformation. Consequently, the loads are directly supportedwithout leading an underlying load path over a stator vane or anassociated exterior carrier beam to the primary structure of themultirotor aircraft.

Moreover, in contrast to conventional stator vanes the support structureof the inventive multirotor aircraft is essentially provided outside ofthe cylindrical duct of each shrouding. Therefore, it may exhibit adrop-like aerodynamic shape that will not block an additional surface inthe downwash area of the rotor or propeller which would, otherwise,lower a desired efficiency of the multirotor aircraft.

As already described above, locations where loads are occurring on andin the shrouding are directly supported and coupled via the carrier beamthat is oriented according to aspects of the present invention with avery stiff load path. This ensures accuracy of shape without any furtherstator vanes, which would be detrimental for noise generation andaerodynamic drag. This is necessary to keep a required airgap betweenthe cylindrical air duct and an associated rotor.

Advantageously, the carrier beam of the inventive multirotor aircraft,which is supported on its both axial sides, shows an improvedvibrational behavior with respect to conventional single cantilever beamsolutions. Both bending and torsional modes are significantly shiftedupward, which is highly beneficial as higher Eigenmodes are usually lesscritical.

Furthermore, advantageously no additional stators are necessary. The atleast one electrical engine is advantageously installed eccentrically toa carrier beam so that respective rotor blades do not cross the engineresp. carrier beam radially. These two measurements largely contributeto lower noise generation of the rotor.

Advantageously, the electrical engine can easily be integrated to thecarrier beam with two suitable ribs. Access for maintenance andinstallation is, thus, guaranteed for at least 180° of the at least oneelectrical engine. This is not feasible with centrically mountedengines, as for these engines the beam would have to be dissolved to aframework structure in order to integrate the engine. This would,however, result to significant drawbacks concerning weight and stiffnessand the access is blocked more or less 360° around the engine, which isnot the case according to the present invention.

Finally, cooling integration is also highly beneficial. Respective heatexchangers can be placed along a given upper flange of the carrier beam.Due to an underlying flight attitude of the multirotor aircraft, thisdoes only marginally increase a projected aerodynamic drag and is wellcovered by the airstream.

Advantageously, the inventive multirotor aircraft is designed fortransportation of passengers and is, in particular, suitable and adaptedfor being certificated for operation within urban areas. It ispreferably easy to fly, has multiple redundancies, meets the safetydemands of the authorities, is cost efficient in design and only createscomparatively low noise. Preferably, the inventive multirotor aircrafthas a comparatively small rotor diameter with a light weight design anda fixed angle of incident and is nevertheless adapted for fulfilment ofan emergency landing, although these rotor characteristics lead to acomparatively low inertia and a non-adjustable torque in operation.

According to one aspect, the inventive multirotor aircraft is capable ofhovering and comprises a distributed propulsion system. It is furtherpreferably designed with autorotation capability, which is necessaryamongst other requirements in order to meet authority regulations, suchas e.g. FAR and EASA regulations, regarding safety failure modes thatamount up to approximately 1*10⁻⁷ failures per flight hour for theentire multirotor aircraft. In the aeronautical sector, these safetylevels are typically defined by the so-called Design Assurance Levels(DAL) A to D.

Preferably, the inventive multirotor aircraft fulfils the authorities'regulation safety level needed to transport passengers. This ispreferentially achieved by a combination and correlation of:

at least two individual rotor assemblies per thrust producing unit,

a redundant, segregated battery layout,

a redundant power supply and harness layout,

a physical separation and segregation of an underlying power management,

redundant, segregated electrical engines, and

pitch control and/or RPM control of the rotor assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are outlined by way of example inthe following description with reference to the attached drawings. Inthese attached drawings, identical or identically functioning componentsand elements are labeled with identical reference numbers and charactersand are, consequently, only described once in the following description.

FIG. 1 shows a side view of a multirotor aircraft with a plurality ofexemplary thrust producing units having shroudings according to oneaspect of the present invention,

FIG. 2 shows a perspective view of a simplified portion of themultirotor aircraft of FIG. 1 illustrating an exemplary shrouding withconstructional details,

FIG. 3 shows a perspective view of a simplified portion of themultirotor aircraft of FIG. 1 illustrating the exemplary shrouding ofFIG. 2 with an electrical engine,

FIG. 4 shows a partly transparent side view of the shrouding of FIG. 2and FIG. 3,

FIG. 5 shows a perspective view of the shrouding of FIG. 4 according toone aspect,

FIG. 6 shows a top view of the shrouding of FIG. 4 and FIG. 5,

FIG. 7 shows exemplary cross-sections of the shrouding of FIG. 4 to FIG.6,

FIG. 8 shows a top view of the shrouding of FIG. 4 to FIG. 6 accordingto a first variant,

FIG. 9 shows a perspective view of a simplified portion of themultirotor aircraft of FIG. 1 with the exemplary shrouding of FIG. 8according to the first variant, and with an electrical engine, and

FIG. 10 shows a perspective view of a simplified portion of themultirotor aircraft of FIG. 1 with an exemplary double shroudingaccording to a second variant, and with two electrical engines.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a multirotor aircraft 1 with an aircraft airframe 2according to the present invention. The aircraft airframe 2 defines asupporting structure that is also referred to hereinafter as the“fuselage” of the multirotor aircraft 1.

The fuselage 2 has an extension in longitudinal direction 1 a, which isillustratively represented by an arrow 1 a that also exemplarilyindicates a forward flight direction of the multirotor aircraft 1, anextension in lateral direction (1 b in FIG. 2, FIG. 3, FIG. 9, and FIG.10), and an extension in vertical direction 1 c. Preferentially, thefuselage 2 is connected to a suitable undercarriage 2 b. Illustratively,the suitable undercarriage 2 b is a skid-type landing gear. However,other suitable undercarriages 2 b, such as e.g. wheel-type landinggears, are likewise contemplated.

Preferably, the fuselage 2 is provided with an outer shell 13 whichdefines an internal volume 2 a that is at least adapted fortransportation of passengers, so that the multirotor aircraft 1 as awhole is adapted for transportation of passengers. The internal volume 2a is preferably further adapted for accommodating operational andelectrical equipment, such as e.g. an energy storage system that isrequired for operation of the multirotor aircraft 1.

It should be noted that exemplary configurations of the internal volume2 a that are suitable for transportation of passengers, but also foraccommodation of operational and electrical equipment, are readilyavailable to the person skilled in the art and generally implemented tocomply with applicable authority regulations and certificationrequirements regarding passenger transportation. Thus, as theseconfigurations of the internal volume 2 a as such are not part of thepresent invention, they are not described in detail for brevity andconciseness.

According to one aspect, the multirotor aircraft 1 comprises a pluralityof thrust producing units 3. Preferably, the plurality of thrustproducing units 3 comprises at least two and preferentially four thrustproducing units 3 a, 3 b, 3 c, 3 d. The thrust producing units 3 a, 3 b,3 c, 3 d are embodied for generating a thrust producing airstream in adirection that is indicated with an arrow 9 in operation, such that athrust illustrated by a thrust vector 9 a is generated, so that themultirotor aircraft 1 is able to hover in the air above a surface 10, asillustrated by way of example. By varying the direction of the thrustvector 9 a, the multirotor aircraft 1 may perform forward, rearward orsideward flight.

The thrust producing units 3 a, 3 b, 3 c, 3 d are structurally connectedto the fuselage 2, as described in detail below with reference to FIG.2. Preferably, at least one of the thrust producing units 3 a, 3 b, 3 c,3 d comprises an associated shrouding in order to improve underlyingaerodynamics and to increase operational safety. By way of example, ashrouding 6 c is associated with the thrust producing unit 3 c and ashrouding 6 d with the thrust producing unit 3 d. The shroudings 6 c, 6d illustratively define a plurality of shroudings 6 and can be made of asimple sheet metal. However, according to one aspect the shroudings 6 c,6 d have a complex geometry, such as e.g. described below with referenceto FIG. 4.

It should be noted that the thrust producing units 3 a, 3 b, 3 c, 3 dare all exemplarily arranged laterally with respect to the fuselage 2,i.e. on the left or right side of the fuselage 2 seen in itslongitudinal direction 1 a. Accordingly, in FIG. 1 only the thrustproducing units 3 c, 3 d are visible, while the thrust producing units 3a, 3 b are masked by the fuselage 2. However, according to one aspectthe thrust producing units 3 a, 3 b are embodied in an axiallysymmetrical manner with respect to the thrust producing units 3 d, 3 c,wherein a longitudinal center axis in the longitudinal direction 1 a ofthe fuselage 2 defines the symmetry axis. Accordingly, only the thrustproducing units 3 c, 3 d and their constituent elements are described inmore detail hereinafter, while a more detailed description of the thrustproducing units 3 a, 3 b is omitted for brevity and conciseness.

It should be noted that this exemplary arrangement is only described byway of example and not for limiting the present invention thereto.Instead, other arrangements are also possible and likewise contemplated.For instance, two of the thrust producing units 3 a, 3 b, 3 c, 3 d canrespectively be arranged at a front and rear section of the fuselage 2,and so on.

According to one aspect, at least one and, preferably, each one of thethrust producing units 3 a, 3 b, 3 c, 3 d is equipped with at least tworotor assemblies. By way of example, the thrust producing unit 3 c isequipped with two rotor assemblies 7 c, 8 c, and the thrust producingunit 3 d is equipped with two rotor assemblies 7 d, 8 d. The rotorassemblies 7 c, 7 d illustratively define a plurality of upper rotorassemblies 7 and the rotor assemblies 8 c, 8 d illustratively define aplurality of lower rotor assemblies 8.

Preferentially, the upper rotor assemblies 7 c, 7 d are arranged abovethe lower rotor assemblies 8 c, 8 d such that the upper and lower rotorassemblies 7 c, 8 c; 7 d, 8 d are stacked, i.e. arranged on top of eachother with congruent rotor axes 12. However, alternative configurationsare likewise contemplated, such as e.g. axially displaced rotor axes.

More specifically, each one of the plurality of upper rotor assemblies 7preferably defines a first rotor axis and each one of the plurality oflower rotor assemblies 8 preferably defines a second rotor axis.Preferentially, the first and second rotor axes are respectivelycongruent as explained above, i.e. coaxially arranged, so that theplurality of upper and lower rotor assemblies 7, 8 define a plurality ofcoaxially arranged rotor axes 12. Illustratively, the upper and lowerrotor assemblies 7 c, 8 c define first and second congruent rotor axes,which are commonly referred to as the rotor axis 12 c, and the upper andlower rotor assemblies 7 d, 8 d define first and second congruent rotoraxes, which are commonly referred to as the rotor axis 12 d. However,other configurations are likewise contemplated. E.g. the rotor axes canbe arranged in parallel to each other, and so on.

The plurality of upper and lower rotor assemblies 7, 8 is preferablypowered by an associated plurality of engines 5, which arepreferentially embodied as electrical engines. Illustratively, the upperand lower rotor assemblies 7 c, 8 c are powered by an electrical engine5 c and the upper and lower rotor assemblies 7 d, 8 d are powered by anelectrical engine 5 d. However, it should be noted that the engines canrespectively be implemented by any suitable engine that is capable ofproducing torque in operation, such as a turbine, diesel engine,Otto-motor, electrical engine and so on.

Preferably, at least one of the upper and lower rotor assemblies 7, 8 isaccommodated inside of a respectively associated shrouding of theplurality of shroudings 6. Illustratively, the lower rotor assemblies 8c, 8 d are accommodated inside of the shroudings 6 c, 6 d, respectively.The upper rotor assemblies 7 c, 7 d are exemplarily located outside of,and in FIG. 1 illustratively above, the shroudings 6 c, 6 d.

FIG. 2 shows the multirotor aircraft 1 of FIG. 1 with the fuselage 2that extends in the longitudinal direction 1 a, the vertical direction 1c, and a lateral direction 1 b. However, in contrast to FIG. 1, thefuselage 2 is merely shown as a truss structure 14 that isillustratively provided with an energy supply 15.

The intension of showing the fuselage 2 as the truss structure 14 is tosimplify illustration of the inventive mounting and attachment of thethrust producing units 3 of FIG. 1 and, more specifically, of theirshroudings 6 to the fuselage 2, as described hereinafter. However, itshould be noted that only the thrust producing unit 3 d with theshrouding 6 d of FIG. 1 is illustrated in greater detail representativefor all thrust producing units 3 and all shroudings 6 of the multirotoraircraft 1 of FIG. 1.

According to one aspect, the shrouding 6 d of the thrust producing unit3 d defines a cylindrical air duct 20 and comprises a leading edgeregion 20 a and a trailing edge region 20 b. Only for clarity, it shouldbe noted that the leading edge region 20 a is the region at the edge ofthe shrouding 6 d, i.e. the cylindrical air duct 20, that is arrangedduring forward flight of the multirotor aircraft 1 in the longitudinaldirection 1 a in an upstream position with respect to the trailing edgeregion 20 b.

The cylindrical air duct 20 is axially delimited by an air inlet region(20 e in FIG. 4) and an air outlet region (20 f in FIG. 4). The leadingedge region 20 a is preferably provided with an additional liftingsurface 27.

According to an aspect as illustrated in FIG. 2, the cylindrical airduct 20 of the shrouding 6 d of the thrust producing unit 3 d isprovided in closed perimeter configuration. In other words, theshrouding 6 d is provided in annular form.

Preferably, a carrier beam 4 e is at least mounted at the leading edgeregion 20 a of the cylindrical air duct 20 to the shrouding 6 d. Thecarrier beam 4 e is preferably at least essentially and, preferentially,completely arranged inside of the cylindrical air duct 20. According tothe present invention, the carrier beam 4 e is oriented at leastessentially, i.e. within predetermined manufacturing tolerances, inparallel to the longitudinal direction 1 a of the fuselage 2. Asillustrated, the carrier beam 4 e is preferably further mounted at thetrailing edge region 20 b of the cylindrical air duct 20 to theshrouding 6 d.

According to one aspect, the carrier beam 4 e is arranged eccentricallyinside of the cylindrical air duct 20 and at least essentially, i.e.within predetermined manufacturing tolerances, coplanar to a crosssection of the cylindrical air duct 20, seen in the vertical direction 1c of the fuselage 2. It should be noted that the eccentric arrangementof the carrier beam 4 e is such that the carrier beam 4 e preferablydoes not cross a rotation center resp. symmetry center of thecylindrical air duct 20.

Preferably, the carrier beam 4 e is bar-shaped and extends from theleading edge region 20 a to the trailing edge region 20 b. According toone aspect, the carrier beam 4 e implements an engine carrier 11 asexplained in more detail below with reference to FIG. 3.

The carrier beam 4 e is preferably connected to the fuselage 2. Thus,the shrouding 6 d resp. the thrust producing unit 3 d is structurallyconnected to the fuselage 2 according to one aspect. More generally, allshroudings 6 resp. all thrust producing units 3 of the multirotoraircraft 1 of FIG. 1 are preferably structurally connected in a similarmanner to the fuselage 2 so that only the structural connection of theshrouding 6 d resp. the thrust producing unit 3 d to the fuselage 2 isexplained in more detail hereinafter for purposes of conciseness andbrevity.

According to one aspect, the shrouding 6 d is configured with asupporting structure 16 that can be made of a simple pressed, bendedmetal sheet, or of a more or less complex structure and material, e.g. afiber reinforced polymer structure. The supporting structure 16preferably embodies an internal volume that can e.g. be used as storagevolume, at least partially, for a battery system of the multirotoraircraft 1.

The supporting structure 16 preferentially encompasses at least partly asuitable structural support 4 that is provided for mounting theshrouding 6 d resp. the thrust producing unit 3 d to the fuselage 2. Asthe shrouding 6 d resp. the thrust producing unit 3 d is illustratedrepresentative for all shroudings 6 resp. thrust producing units 3 asexplained above, this means that each one of the shroudings 6 resp.thrust producing units 3 is preferably mounted to the fuselage 2 bymeans of an associated structural support that is embodied similar tothe structural support 4.

The structural support 4 and, thus, the shrouding 6 d preferablycomprises a forward beam 4 a and an aft beam 4 b, as well as the carrierbeam 4 e. The forward beam 4 a and the aft beam 4 b preferentiallyconnect the carrier beam 4 e to the fuselage 2. The forward beam 4 a andthe aft beam 4 b are preferably integrally formed with the carrier beam4 e or at least rigidly attached thereto.

According to one aspect, the forward beam 4 a and the aft beam 4 b arearranged outside of the cylindrical air duct 20. Preferably, the forwardbeam 4 a and the aft beam 4 b are arranged inside of the shrouding 6 d,i.e. inside of the supporting structure 16.

Illustratively, the part of the shrouding 6 d which is provided with theforward beam 4 a and the aft beam 4 b that are connected to the carrierbeam 4 e defines an inner portion of the cylindrical air duct 20, i.e. aportion that is adjacent to the fuselage 2. The remaining portion of theshrouding 6 d is hereinafter referred to as an “outer half of duct” andlabeled with the reference sign 21. This outer half of duct 21advantageously provides additional lift in operation of the multirotoraircraft 1.

Preferably, the forward beam 4 a comprises a forward flange 4 c and theaft beam 4 b comprises an aft flange 4 d. The forward flange 4 c and theaft flange 4 d are preferentially rigidly mounted to the fuselage 2.

The forward beam 4 a and the forward flange 4 c are preferablyintegrally formed or at least rigidly mounted to each other. Similarly,the aft beam 4 b and the aft flange 4 d are preferably integrally formedor at least rigidly mounted to each other. Furthermore, the forwardflange 4 c and the aft flange 4 d may be integrally formed or at leastbe rigidly mounted to each other.

According to one aspect, the forward flange 4 c and the aft flange 4 dare connected to an associated flange 4 f of the fuselage 2. Preferably,the forward flange 4 c and the aft flange 4 d are rigidly but removablymounted to the flange 4 f. The flange 4 d may be integrally formed withthe fuselage 2 or at least be rigidly mounted thereto.

The forward flange 4 c and the aft flange 4 d may be rigidly mounted toor integrally formed with a respective forward beam and aft beamassociated with the shrouding 6 a of the thrust producing unit 3 a.However, the shrouding 6 a may likewise be provided with separateforward and aft flanges that are similar to the forward and aft flanges4 c, 4 d of the shrouding 6 d of the thrust producing unit 3 d and whichare, in turn, mounted to the flange 4 f of the fuselage 2.

A more specific exemplary realization of the shrouding 6 d of the thrustproducing unit 3 d as such is described in more detail below withreference to FIG. 4 through FIG. 8.

FIG. 3 shows the multirotor aircraft 1 with the fuselage 2 of FIG. 1 andFIG. 2 with the shrouding 6 d of the thrust producing unit 3 d that isprovided in closed perimeter configuration as described above withreference to FIG. 2. It should be noted that the thrust producing unit 3d with the shrouding 6 d is again illustrated representative for allthrust producing units 3 resp. shroudings 6 and attached to the flange 4f of the fuselage 2, as described above with reference to FIG. 2.Furthermore, similar to the illustration in FIG. 2, the fuselage 2 isshown as the truss structure 14 that accommodates the energy supply 15.

According to one aspect, the shrouding 6 d is provided with the enginecarrier 11 that is embodied by the carrier beam 4 e of FIG. 2. The atleast one electrical engine 5 d of FIG. 1 is preferably mounted to thecarrier beam 4 e resp. the engine carrier 11, as illustrated.

Preferably, the electrical engine 5 d is eccentrically mounted to thecarrier beam 4 e resp. the engine carrier 11. More specifically, asexplained above with reference to FIG. 2, the engine carrier 11 ismounted eccentrically inside of the cylindrical air duct 20 resp. theshrouding 6 d. In other words, the engine carrier 11 is mounted to theshrouding 6 d such that it is arranged in parallel to a center lineresp. symmetry axis 18 that traverses the shrouding 6 d from the leadingedge region 20 a to the trailing edge region 20 b, but without crossingthe rotation axis 12 d of the electrical engine 5 d. The center line 18,however, crosses the rotation axis 12 d and is oriented in parallel tothe longitudinal direction 1 a of the fuselage 2, at least essentially.

It should be noted that the eccentrical mounting of the electricalengine 5 d to the engine carrier 11 refers to a mounting of theelectrical engine 5 d to the engine carrier 11 such that the electricalengine 5 d is only laterally in contact with the engine carrier 11, i.e.that a lateral position 19 of the electrical engine 5 d is mounted tothe engine carrier 11. In other words, only one side of the electricalengine 5 d is in connection with the engine carrier 11. Accordingly, theelectrical engine 5 d has a periphery that is essentially available forcooling of the electrical engine 5 d in operation.

FIG. 4 shows a schematic view of the shrouding 6 d of the thrustproducing unit 3 d of FIG. 2 and FIG. 3, which defines the cylindricalair duct 20, for illustrating an aerodynamically improved configurationthereof according to one aspect of the present invention.Illustratively, the cylindrical air duct 20 is radially delimited by thesupporting structure 16 of FIG. 2 and FIG. 3.

The cylindrical air duct 20 is preferably axially delimited by an airinlet region 20 e and an air outlet region 20 f. Outside of thecylindrical air duct 20 and preferably above as well as adjacent to theair inlet region 20 e of the cylindrical air duct 20 is preferablyarranged the first rotor assembly 7 d of FIG. 1.

It should be noted that the air duct 20 is only by way of exampledesignated as a “cylindrical” air duct and not for limiting the presentinvention accordingly. In other words, while a “cylindrical” shaping ofthe air duct implies equal radii all along the air duct 20 from the airinlet region 20 e to the air outlet region 20 f, alternativeconfigurations are likewise contemplated. For instance, the air duct 20may exhibit the form of a frustum, such that its radius is e.g. greaterat the air outlet region 20 f than at the air inlet region 20 e, and soon. Therefore, is should be understood that the expression “cylindricalair duct” is meant to encompass also such alternative configurations ofthe air duct 20.

The air inlet region 20 e preferably exhibits in circumferentialdirection of the cylindrical air duct 20 an undulated geometry. Morespecifically, this undulated geometry implies that when moving incircumferential direction of the cylindrical air duct 20 along the airinlet region 20 e, an undulated motion resp. a wave-shaped movement isperformed.

In operation of the thrust producing unit 3 d, the air inlet region 20 epreferably functions as an air collector and is, therefore, hereinafteralso referred to as the “collector 20 e” of the cylindrical air duct 20,for simplicity and clarity. The air outlet region 20 f may, but notnecessarily, be embodied and function as a diffusor and is thereforehereinafter also referred to as the “diffusor 20 f” of the cylindricalair duct 20, for simplicity and clarity.

The cylindrical air duct 20 and, more particularly, the shrouding 6 d,comprises the leading edge region 20 a and the trailing edge region 20 bof FIG. 2 and FIG. 3. Furthermore, the shrouding 6 d, i.e. thecylindrical air duct 20, preferentially comprises a board side lateralregion 20 c and a star board side lateral region 20 d that are locatedat the air inlet region 20 e.

More specifically, the leading edge region 20 a is diametrically opposedto the trailing edge region 20 b in circumferential direction of theshrouding 6 d, i.e. the cylindrical air duct 20, and the board sidelateral region 20 c is diametrically opposed to the star board sidelateral region 20 d. Furthermore, the board side lateral region 20 c andthe star board side lateral region 20 d are respectively arrangedbetween the leading edge region 20 a and the trailing edge region 20 bin circumferential direction of the shrouding 6 d, i.e. the cylindricalair duct 20.

According to one aspect, the cylindrical air duct 20 has a heightdefined between the diffusor 20 f and the collector 20 e in axialdirection of the cylindrical air duct 20 that varies in circumferentialdirection of the cylindrical air duct 20. This varying height definesthe undulated geometry of the collector 20 e as described hereinafter.

More specifically, a height 24 a at the leading edge region 20 a ispreferably smaller than a height 24 c at the board side lateral region20 c and/or the star board side lateral region 20 d. Furthermore, aheight 24 b at the trailing edge region 20 b is preferably smaller thanthe height 24 c at the board side lateral region 20 c and/or the starboard side lateral region 20 d. Moreover, the height 24 b at thetrailing edge region 20 b is preferably smaller than the height 24 a atthe leading edge region 20 a. According to one aspect, the height 24 cat the board side lateral region 20 c and/or the star board side lateralregion 20 d is selected in a range from 0.05*D to 0.5*D, wherein Ddefines a diameter, preferably an inner diameter (20 g in FIG. 6), ofthe cylindrical air duct 20.

According to one aspect, the collector 20 e of the cylindrical air duct20 has a radius that varies in the circumferential direction of thecylindrical air duct 20. In other words, the collector 20 e ispreferably not provided with a flat upper edge, i.e. its edge thatpoints away from the diffusor 20 f, but with a rounded upper edge.Preferentially, the radius of the collector 20 e, which is hereinafteralso referred to as the “collector radius” for simplicity and clarity,differs between at least two of the leading edge region 20 a, thetrailing edge region 20 b, the board side lateral region 20 c and thestar board side lateral region 20 d.

Preferably, a collector radius 25 a at the leading edge region 20 a isselected in a range from 0.01*D to 0.25*D, a collector radius 25 b atthe trailing edge region 20 b is selected in a range from 0 to 0.25*D,and a collector radius 25 c at the board side lateral region 20 c and/orthe star board side lateral region 20 d is selected in a range from0.01*D to 0.25*D. As already mentioned above, D defines the diameter,preferably the inner diameter (20 g in FIG. 6), of the cylindrical airduct 20.

Likewise, the diffusor 20 f of the cylindrical air duct 20 may have aradius that varies in the circumferential direction of the cylindricalair duct 20. In other words, the diffusor 20 f is not necessarilyprovided as illustrated with a flat lower edge, i.e. its edge thatpoints away from the collector 20 e, but with a rounded lower edge.Preferentially, the radius of the diffusor 20 f, which is hereinafteralso referred to as the “diffusor radius” for simplicity and clarity,differs between at least two of the leading edge region 20 a, thetrailing edge region 20 b, the board side lateral region 20 c and thestar board side lateral region 20 d.

Preferably, a diffusor radius 26 a at the leading edge region 20 a isselected in a range from 0 to 0.1*D, a diffusor radius 26 b at thetrailing edge region 20 b is selected in a range from 0 to 0.1*D, and adiffusor radius 26 c at the board side lateral region 20 c and/or thestar board side lateral region 20 d is selected in a range from 0 to0.1*D. Again, as already mentioned above, D defines the diameter,preferably the inner diameter (20 g in FIG. 6), of the cylindrical airduct 20.

FIG. 5 shows the shrouding 6 d of FIG. 4 that defines the cylindricalair duct 20, which is preferably axially delimited by the collector 20 eand the diffusor 20 f and which comprises the leading edge region 20 a,the trailing edge region 20 b, the board side lateral region 20 c andthe star board side lateral region 20 d. The leading edge region 20 a isprovided with the additional lifting surface 27.

FIG. 6 shows the shrouding 6 d of FIG. 2 to FIG. 5 that defines thecylindrical air duct 20, which comprises the leading edge region 20 a,the trailing edge region 20 b, the board side lateral region 20 c andthe star board side lateral region 20 d according to FIG. 4 and FIG. 5.Illustratively, a diameter and, more specifically, an inner diameter Dof the cylindrical air duct 20 is labeled with the reference sign 20 g.Furthermore, the azimuth ψ of the cylindrical air duct 20, i.e. theshrouding 6 d, is labeled with the reference sign 20 h. By way ofexample, it is assumed that the azimuth ψ is defined in clockwisedirection of the shrouding 6 d as illustrated and starts turning fromthe trailing edge region 20 b such that ψ=0 at the trailing edge region20 b.

FIG. 7 shows four exemplary cross-sections of the shrouding 6 d of FIG.4 to FIG. 6 that defines the cylindrical air duct 20, which ispreferably axially delimited by the collector 20 e and the diffusor 20 fand which comprises the leading edge region 20 a, the trailing edgeregion 20 b, the board side lateral region 20 c and the star board sidelateral region 20 d. Each cross-section corresponds to a sectional viewof the shrouding 6 d at a given azimuth ψ of FIG. 6.

More specifically, a first sectional view illustrates an exemplarycross-section of the shrouding 6 d at the azimuth ψ*=180° seen indirection of the cut line A-A of FIG. 6. This first sectional viewillustrates the leading edge region 20 a of the shrouding 6 d that isprovided with the additional lifting surface 27. By way of example, thecollector 20 e is provided at the leading edge region 20 a as describedabove with reference to FIG. 4 with a rounded upper edge, while thediffusor 20 f is illustratively provided with a flat lower edge.

A second sectional view illustrates an exemplary cross-section of theshrouding 6 d at the azimuth ψ=0° seen in direction of the cut line A-Aof FIG. 6. This second sectional view illustrates the trailing edgeregion 20 b of the shrouding 6 d. By way of example and as describedabove with reference to FIG. 4, the collector 20 e is provided at thetrailing edge region 20 b with a rounded upper edge and the diffusor 20f is provided with a rounded lower edge.

A third sectional view illustrates an exemplary cross-section of theshrouding 6 d at the azimuth ψ=90° seen in direction of the cut line B-Bof FIG. 6. This third sectional view illustrates the board side lateralregion 20 c of the shrouding 6 d. By way of example, the collector 20 eis provided at the board side lateral region 20 c as described abovewith reference to FIG. 4 with a rounded upper edge, while the diffusor20 f is illustratively provided with a flat lower edge.

A fourth sectional view illustrates an exemplary cross-section of theshrouding 6 d at the azimuth ψ*=270° seen in direction of the cut lineB-B of FIG. 6. This fourth sectional view illustrates the star boardside lateral region 20 d of the shrouding 6 d. By way of example, thecollector 20 e is provided at the star board side lateral region 20 d asdescribed above with reference to FIG. 4 with a rounded upper edge,while the diffusor 20 f is illustratively provided with a flat loweredge.

FIG. 8 shows the shrouding 6 d of FIG. 4 to FIG. 7 that defines thecylindrical air duct 20, which comprises the leading edge region 20 a,the trailing edge region 20 b, the board side lateral region 20 c andthe star board side lateral region 20 d. However, in contrast to theimplementation of the shrouding 6 d according to FIG. 4 to FIG. 7, thetrailing edge region 20 b of the cylindrical air duct 20 is now at leastessentially open. Preferably, the cylindrical air duct 20 is open at thetrailing edge region 20 b over a predetermined opening angle 28 of e.g.30° to 180°. In other words, the cylindrical air duct 20 is providedwith a shrouding opening that is defined by the predetermined openingangle 28 and, therefore, hereinafter also referred to by using thereference sign 28.

FIG. 9 shows the multirotor aircraft 1 with the fuselage 2 of FIG. 1with the shrouding 6 d of the thrust producing unit 3 d that is, incontrast to FIG. 2 and FIG. 3, now provided in opened perimeterconfiguration. In the opened perimeter configuration, the shrouding 6 dis at least partly cut-off at the trailing edge region 20 b of thecylindrical air duct 20 over the predetermined opening angle 28 of FIG.8, as described above with reference to FIG. 8.

It should be noted that the opened perimeter configuration isadvantageous with respect to the closed perimeter configurationdescribed above with reference to FIG. 2 and FIG. 3, as it allows tofurther reduce the undesired aerodynamical drag on the shrouding 6 d. Infact, a majority of the aerodynamical drag at the shrouding 6 daccording to FIG. 2 and FIG. 3 in closed perimeter configuration iscreated at the trailing edge region 20 b of the cylindrical air duct 20.Thus, by cutting-off the trailing edge region 20 b over thepredetermined opening angle 28, the aerodynamical drag can be reducedsignificantly.

Illustratively, and only by way of example and not for limiting thepresent invention accordingly, the predetermined opening angle 28amounts approximately up to 180°. In other words, in the illustratedexample, the trailing edge region 20 b of the shrouding 6 d as shown inFIG. 1 to FIG. 3 is completely cut-off. Preferably, the trailing edgeregion 20 b of the shrouding 6 d as shown in FIG. 1 to FIG. 3 isreplaced by two aerodynamically shaped longitudinal extensions 22 a, 22b. The latter are exemplarily embodied as lateral continuations of thestar board side lateral region 20 d and the board side lateral region 20c, respectively, of FIG. 4.

According to one aspect, the thrust producing unit 3 d resp. theshrouding 6 d is again provided with the engine carrier 11. However, incontrast to FIG. 2 and FIG. 3, the engine carrier 11 is no moreimplemented by means of the carrier beam 4 e of FIG. 2 and FIG. 3, whichis mounted at the leading edge region 20 a and the trailing edge region20 b of the cylindrical air duct 20 to the shrouding 6 d, as thetrailing edge region 20 b is cut-off. Instead, the carrier beam 4 e isreplaced by a cantilever 17, which is only mounted at the leading edgeregion 20 a of the cylindrical air duct 20 to the shrouding 6 d andwhich now embodies the engine carrier 11.

Accordingly, the at least one electrical engine 5 d of FIG. 1 is nowpreferably mounted to the cantilever 17 resp. the engine carrier 11, asillustrated. Preferentially, the electrical engine 5 d is eccentricallymounted to the cantilever 17 resp. the engine carrier 11 of the thrustproducing unit 3 d.

It should be noted that the thrust producing unit 3 d with the shrouding6 d is again illustrated representative for all thrust producing units 3resp. shroudings 6 and attached to the flange 4 f of the fuselage 2, asdescribed above with reference to FIG. 2. Furthermore, similar to theillustration in FIG. 2, the fuselage 2 is shown as the truss structure14 that accommodates the energy supply 15.

Moreover, it should be noted that the shrouding 6 d and, moreparticularly, the leading edge region 20 a of the shrouding 6 d as wellas a front portion of the flange 4 f of the fuselage 2 resp. thestructural support 4 is now prone to comparatively high bending forces,which could no more be compensated by means of the trailing edge region20 b. In fact, as the trailing edge region 20 b is cut-off, loadtransfer from the at least one electrical engine 5 d via the supportstructure 4 and, more particularly, at least in part via the aft beam 4b and the aft flange 4 d of FIG. 2 is no more possible in the openedperimeter configuration. Thus, another compensation means is required inorder to guarantee a reliable and secure operation of the thrustproducing unit 3 d.

As a consequence, according to one aspect the shrouding 6 d is not onlyprovided with the additional lifting surface 27 at its leading edgeregion 20 a, but also with a torque box 23 that is configured to supporteven higher bending forces. A cut view along a cut line A-A shows anexemplary implementation resp. aerodynamic profile of the torque box 23,which is preferably implemented wing-like.

Advantageously, the flange 4 f at the fuselage 2 respectively thestructural support 4 that is mounted to the flange 4 f at the fuselage 2may also be provided with the torque box 23. In other words, the torquebox 23 extends from the shrouding 6 d to the structural support 4 sothat an even better bending force compensation may be achieved byenlarging the torque box 23 further.

FIG. 10 shows the multirotor aircraft 1 with the fuselage 2 that is onlyillustratively implemented by means of the truss structure 14 and thatcomprises the shrouding 6 d of the thrust producing unit 3 d of FIG. 9,which is provided in opened perimeter configuration. However, incontrast to FIG. 9, the thrust producing unit 3 d is now provided as aninterconnected, double thrust producing unit, i.e. as a cascaded thrustproducing unit having exemplarily two interconnected resp. cascadedshroudings that now form the shrouding 6 d. More specifically, theshrouding 6 d now exhibits in top view an E-shaped form, while in FIG. 9the shrouding 6 d exhibits in top view a C-shaped form.

In fact, according to one aspect the cascaded E-shaped shrouding 6 d ofFIG. 10 merely consists of two single cascaded C-shaped shroudings 6 daccording to FIG. 9, which are interconnected by means of a suitableinterconnection area 6 e, such that they are arranged laterally withrespect to each other.

Preferably, the leading edge regions 20 a of both laterally arrangedresp. cascaded shroudings that form the shrouding 6 d and theinterconnection area 6 e, as well as the structural support 4 that ismounted to the flange 4 f of the fuselage 2, are now embodying thetorque box 23 of FIG. 9. Furthermore, each one of the C-shapedshroudings that form the E-shaped cascaded shrouding 6 d is now providedwith an associated engine carrying cantilever 17 that is connected to anassociated one of the electrical engines 5.

However, it should be noted that although the shrouding 6 d according toFIG. 10 only shows two laterally arranged resp. cascaded shroudings thatform the shrouding 6 d and that define the thrust producing unit 3 d,more than two laterally arranged shroudings that form the shrouding 6 dare likewise contemplated. Furthermore, it should again be noted thatthe shrouding 6 d is merely represented by way of example andrepresentative for all shroudings 6 of FIG. 1.

Finally, it should be noted that modifications of the above describedaspects of the present invention are also within the common knowledge ofthe person skilled in the art and, thus, also considered as being partof the present invention. By way of example, the thrust producing units3 a and 3 d of FIG. 1 may be implemented as cascaded thrust producingunits with E-shaped shroudings as described with reference to FIG. 10,while the thrust producing units 3 b, 3 c of FIG. 1 may be embodied assingle C-shaped shroudings as explained above with reference to FIG. 9.Furthermore, it should be noted that also the thrust producing unitshaving shroudings in closed perimeter configuration as described abovewith reference to FIG. 2 and FIG. 3 may be embodied as cascaded thrustproducing units, wherein two or more laterally arranged shroudingsaccording to FIG. 10 are implemented. In other words, FIG. 10 may berealized by using shroudings in closed perimeter configuration insteadof using shroudings in opened perimeter configuration. Finally, itshould also be noted that on a single multirotor aircraft, such as themultirotor aircraft 1 of FIG. 1, a mix of shroudings in closed perimeterconfiguration and open perimeter configuration may be used. Also, in aconfiguration such as illustrated in FIG. 10, wherein cascadedshroudings are used, one or more of the shroudings may be implemented inopened perimeter configuration while one or more other shroudings areimplemented in closed perimeter configuration. For instance, in a thrustproducing unit with three shroudings, a middle shrouding may be embodiedin opened perimeter configuration while both outer shroudings areembodied in closed perimeter configuration, and vice versa.

REFERENCE LIST

-   1 Multirotor aircraft-   1 a Aircraft longitudinal direction and forward flight direction-   1 b Aircraft lateral direction-   1 c Aircraft vertical direction-   2 Aircraft airframe-   2 a Aircraft airframe internal volume-   2 b Undercarriage-   3 Thrust producing units-   3 a, 3 b, 3 c, 3 d Thrust producing unit-   4 Thrust producing units structural support-   4 a Forward beam-   4 b Aft beam-   4 c Forward flange-   4 d Aft flange-   4 e Engine carrier beam-   4 f Flange-   5 Engines-   5 a, 5 c, 5 d Electrical engine-   6 Shrouding units-   6 a, 6 c, 6 d Shrouding-   6 e Shrouding interconnection area-   7 Upper rotor assemblies-   7 c, 7 d Upper rotor assembly-   8 Lower rotor assemblies-   8 c, 8 d Lower rotor assembly-   9 Thrust producing airstream direction-   9 a Thrust vector-   10 Ground-   11 Engine carrier-   12 Rotor axes-   12 c, 12 d Rotor axis-   13 Outer shell-   14 Truss structure-   15 Energy supply-   16 Supporting structure-   17 Engine carrier cantilever-   18 Center line from leading edge to trailing edge-   19 Lateral position-   20 Air duct-   20 a Leading edge region-   20 b Trailing edge region-   20 c Board side lateral region-   20 d Star board side lateral region-   20 e Collector-   20 f Diffusor-   20 g Air duct inner diameter (D)-   20 h Air duct azimuth (ψ)-   Outer half of duct-   22 a, 22 b Longitudinal extensions-   23 Torque box-   24 a Total height of air duct leading edge (HL)-   24 b Total height of air duct trailing edge (HT)-   24 c Total height of air duct lateral region (HS)-   25 a Collector radius at air duct leading edge (CRL)-   25 b Collector radius at air duct trailing edge (CRT)-   25 c Collector radius at air duct lateral region (CRS)-   26 a Diffusor radius at air duct leading edge (DRL)-   26 b Diffusor radius at air duct trailing edge (DRT)-   26 c Diffusor radius at air duct lateral region (DRS)-   27 Additional lifting surface-   28 Shrouding opening and opening angle

What is claimed is:
 1. A multirotor aircraft with an airframe thatextends in a longitudinal direction, and with at least one thrustproducing unit for producing thrust in a predetermined thrust direction,wherein the at least one thrust producing unit comprises a shroudingthat is associated with at least one rotor assembly comprising at leastone electrical engine, wherein the shrouding defines a cylindrical airduct that is axially delimited by an air inlet region and an air outletregion, wherein a cantilever is mounted at a leading edge region of thecylindrical air duct to the shrouding such that the cantilever isarranged inside of the cylindrical air duct and oriented at leastessentially in parallel to the longitudinal direction, wherein theshrouding comprises a forward beam which connects the cantilever to theairframe, the forward beam being arranged outside of the cylindrical airduct and comprising a forward flange that is rigidly attached to theairframe, wherein the at least one electrical engine is mounted to thecantilever, and wherein the cylindrical air duct is provided in openedperimeter configuration, the shrouding being at least partly cut-off inthe opened perimeter configuration at a trailing edge region of thecylindrical air duct over a predetermined opening angle.
 2. Themultirotor aircraft of claim 1, wherein the cantilever is arrangedeccentrically inside of the cylindrical air duct and at leastessentially coplanar to a cross section of the cylindrical air duct. 3.The multirotor aircraft of claim 1, wherein the cantilever is bar-shapedand extends from the leading edge region in direction of the cut-offtrailing edge region.
 4. The multirotor aircraft claim 1, wherein theforward beam and the forward flange are integrally formed.
 5. Themultirotor aircraft of claim 1, wherein the shrouding is provided withan additional lifting surface at the leading edge region of thecylindrical air duct.
 6. The multirotor aircraft of claim 1, wherein theat least one electrical engine is eccentrically mounted to thecantilever.
 7. The multirotor aircraft of claim 1, wherein the air inletregion exhibits in circumferential direction of the cylindrical air ductan undulated geometry, wherein the cylindrical air duct comprises incircumferential direction the leading edge region and the trailing edgeregion that is diametrically opposite of the leading edge region, aboard side lateral region and a diametrically opposed star board sidelateral region, wherein the board side lateral region and the star boardside lateral region are respectively arranged in the circumferentialdirection of the cylindrical air duct between the leading edge regionand the trailing edge region, and wherein a maximum height of the crosssection at the leading edge region is smaller than a maximum height ofthe cross section at the board side lateral region and/or the star boardside lateral region.
 8. The multirotor aircraft of claim 7, wherein thecylindrical air duct exhibits a maximum height of the cross sectiondefined between the air outlet region and the air inlet region in axialdirection of the cylindrical air duct that varies in circumferentialdirection of the cylindrical air duct, wherein the maximum height of thecross section that varies in the circumferential direction of thecylindrical air duct defines the undulated geometry of the air inletregion.
 9. The multirotor aircraft of claim 7, wherein the maximumheight of the cross section at the trailing edge region is smaller thana maximum height of the cross section at the board side lateral regionand/or the star board side lateral region.
 10. The multirotor aircraftof claim 7, wherein the maximum height of the cross section at thetrailing edge region is smaller than the maximum height of the crosssection at the leading edge region.
 11. The multirotor aircraft of claim1, wherein the cantilever is mounted only at the leading edge region ofthe cylindrical air duct to the shrouding.
 12. The multirotor aircraftof claim 1, wherein the shrouding includes a torque box at the leadingedge region of the cylindrical air duct, the torque box being capable ofcompensating bending forces.
 13. A multirotor aircraft comprising: anairframe extending in a longitudinal direction; and at least one thrustproducing unit capable of producing thrust in a predetermined thrustdirection; the at least one thrust producing unit comprising: at leastone rotor assembly comprising at least one electrical engine; ashrouding associated with the at least one rotor assembly, the shroudingdefining a cylindrical air duct axially delimited by an air inlet regionand an air outlet region, a cantilever mounted at a leading edge regionof the cylindrical air duct to the shrouding such that the cantilever isarranged inside of the cylindrical air duct and oriented at leastessentially in parallel to the longitudinal direction, the shroudingcomprising a forward beam connecting the cantilever to the airframe, theforward beam disposed outside of the cylindrical air duct and comprisinga forward flange rigidly attached to the airframe, the at least oneelectrical engine being mounted to the cantilever, and wherein thecylindrical air duct is provided in opened perimeter configuration, theshrouding being at least partly cut-off in the opened perimeterconfiguration at a trailing edge region of the cylindrical air duct overa predetermined opening angle.
 14. The multirotor aircraft of claim 13,wherein the cantilever is arranged eccentrically inside of thecylindrical air duct and is essentially coplanar to a cross section ofthe cylindrical air duct, and wherein the at least one electrical engineis eccentrically mounted to the cantilever, and wherein the cantileveris bar-shaped and extends from the leading edge region in direction ofthe cut-off trailing edge region.
 15. The multirotor aircraft claim 13,wherein the forward beam and the forward flange are integrally formed.16. The multirotor aircraft of claim 13, wherein the shrouding includesan additional lifting surface at the leading edge region of thecylindrical air duct.
 17. The multirotor aircraft of claim 13, whereinthe air inlet region exhibits in circumferential direction of thecylindrical air duct an undulated geometry, wherein the cylindrical airduct comprises in circumferential direction the leading edge region andthe trailing edge region that is diametrically opposite the leading edgeregion, a board side lateral region and a diametrically opposed starboard side lateral region, wherein the board side lateral region and thestar board side lateral region are respectively arranged in thecircumferential direction of the cylindrical air duct between theleading edge region and the trailing edge region, and wherein a maximumheight of the cross section at the leading edge region is smaller than amaximum height of the cross section at the board side lateral regionand/or the star board side lateral region.
 18. The multirotor aircraftof claim 17, wherein the cylindrical air duct exhibits a maximum heightof the cross section defined between the air outlet region and the airinlet region in axial direction of the cylindrical air duct that variesin circumferential direction of the cylindrical air duct, wherein themaximum height of the cross section that varies in the circumferentialdirection of the cylindrical air duct defines the undulated geometry ofthe air inlet region.
 19. The multirotor aircraft of claim 17, whereinthe maximum height of the cross section at the trailing edge region issmaller than a maximum height of the cross section at the board sidelateral region and/or the star board side lateral region.
 20. Themultirotor aircraft of claim 13, wherein the cantilever is mounted onlyat the leading edge region of the cylindrical air duct to the shrouding,and wherein the shrouding includes a torque box at the leading edgeregion of the cylindrical air duct, the torque box being capable ofcompensating bending forces.